What you do is right click on the enemy you want to be the boss and change the part which says
IsObjective from 0 to 1.
Now when the enemy dies the level will end. If you want to make sure that a number of enemies need to die then add one to each enemy, so if 2 enemies need to die, one will read
IsObjective 1, and the other will read is
IsObjective 2, and so on.
attached to this post is an fpm with stock media, you only have to kill the zombie in the final room to win the level. Just copy it to your mapbank and open it, run test game and you'll see.